Exhaust systems for engines can help control the amount of pollution produced by the engine. One form of exhaust treatment can include a catalytic converter having a selective catalytic reduction catalyst. Selective catalytic reduction (SCR) is a process that can be used to control nitrogen oxide (NOx) emissions. SCR is a way of converting NOx into diatomic nitrogen (N2) and water (H2O) in a catalytic converter, with the help of an ammonia-based reductant and a catalyst.
Ammonia-based NOx reduction can include using urea ((NH2)2Co), or another ammonia-based reductant. Urea can be provided in the form of a second fluid such as diesel exhaust fluid, or DEF. Diesel exhaust fluid may include an aqueous urea solution made with, for example, a 32.5% urea and 67.5% deionized water.
While urea SCR works, the exhaust systems and methods that rely on urea, place a burden on the user. The user must purchase urea (e.g. diesel exhaust fluid) and carry it in a tank onboard the vehicle. When the diesel exhaust fluid runs out, the SCR system will no longer work properly. If the user does not fill the tank with diesel exhaust fluid, the beneficial effects of the SCR system may be degraded or eliminated. Another downside of using a compound including urea is that it freezes at about 12 degrees Fahrenheit, so it may require heated storage, especially in colder climates.
While urea does present some issues for the user, urea can successfully reduce NOx emissions. When urea is injected into the exhaust stream during the SCR process, the urea is broken down into ammonia (NH3) and carbon dioxide (CO2) in the presence of a catalyst. The ammonia then reacts with the NO to create nitrogen (N2) and water vapor (H2O), which can be less troublesome emissions compared to NOx.
The disclosed systems and method are directed to overcoming one or more of the problems set forth above.